1. Field of the Invention
The invention relates to a hydrogen generation device that alternately generates hydrogen through a reforming reaction using a catalyst, and recovers the catalyst via a recovery reaction. The recovery reaction recovers the catalyst by heating the catalyst for the next reforming reaction. The invention also relates to a fuel cell system that includes the hydrogen generation device.
2. Description of the Related Art
In electric vehicles, a fuel cell is provided as a power source. Hydrogen used to generate electric power in the fuel cell, or reactant used to generate hydrogen is provided.
In the case where hydrogen itself is provided, it may be stored in the following ways: (i) the hydrogen gas is compressed, and stored in a high-pressure container, (ii) the hydrogen gas is liquefied and stored in a tank, or (iii) the hydrogen is provided using hydrogen storage alloy or hydrogen adsorbent. When hydrogen gas is stored in a high-pressure container as described in (i), only a small amount of hydrogen can be stored in the high-pressure container because of the thick walls and small internal volume of the container. When liquefied hydrogen is stored as described in (ii), a portion of the liquefied hydrogen is lost due to vaporization, and a great amount of energy is expended in liquefying hydrogen. When hydrogen is provided using the hydrogen storage alloy or the hydrogen adsorbent as described in (iii), the density of stored hydrogen is generally lower than that needed to power an electric vehicle. Further, controlling storage, adsorption, and the like of hydrogen is difficult. In the case where the reactant is provided, hydrogen gas may be generated through a steam-reforming reaction using the reactant. However, because the reforming reaction is an endothermic reaction, a heat source needs to be provided. Providing an electric heater or the like as the heat source in the system decreases the energy efficiency of the entire system. In addition, the system must also be able to extract the amount of hydrogen needed to power the vehicle under the various environmental conditions in which the vehicle operates.
Currently, a method for supplying hydrogen has not been technically established. However, because it is predicted that hydrogen will be used in the increasing number of devices, the method of supplying hydrogen needs to be established.
As a technology related to the above, US Patent Application Publication No. 2003-0235529 describes a reforming device that generates hydrogen by alternately carrying out a steam-reforming reaction and a recovery reaction. The steam-reforming reaction is an endothermic reaction, and is carried out using reactant on a catalyst under specified conditions. The recovery reaction is used to raise the temperature of the catalyst, which has decreased as a result of the steam-reforming reaction.
In the reforming device for generating hydrogen, a pair of catalysts is used. When the reforming reaction is carried out on one of the catalysts, the recovery reaction is carried out on the other catalyst. The reforming reaction and the recovery reaction are carried out alternately on each catalyst. This helps to maintain a consistent level of performance of the reforming reaction. The reforming device is particularly suitable for reforming liquid fuel or the like from which the sulfur components have not been completely removed.
For example, US Patent Application Publication No. 2004-0170558, No. 2004-0170559, and No. 2004-0175326 also describe technologies related to the above. Also, Japanese Patent Application Publication No. 2004-146337 describes a fuel cell that includes a hydrogen-permeable material, and generates electric power in a high temperature range.
Because the reforming reaction is most efficient at high temperatures, the temperatures of the two or more catalysts need to be increased. Therefore, when the catalysts are cold, for example, when the reforming device is started, all the catalysts need to be heated. As a result, much time is required to bring the temperatures of all the catalysts to the appropriate temperature for generating the amount of hydrogen that is needed. That is, the reforming device cannot be activated in a short time. When the amount of hydrogen needed sharply increases due to a sharp increase in the demand for electric power, for example, when the load on the HMFC 30 increases while the fuel cell is generating electric power, the reforming device cannot generate the additional hydrogen required immediately.
Particularly in the case where a fuel cell system that includes the fuel cell is provided in a vehicle, for example, a large-sized electric power storage device needs to be provided to supply electric power before the fuel cell starts to generate electric power. The power storage device is needed because the amount of hydrogen gas that is generated by the reforming device and supplied to the fuel cell is initially insufficient to generate the electric power required. However, the additional components not only increase the weight of the fuel cell system, but the cost of the system as well. Also, it is difficult to supply electric power stably using the large-sized electric power storage device. Further, the large-sized electric power storage device occupies a large space in the vehicle.